Power consumption of integrated circuits is a concern in battery operated portable system designs. With technology scaling, the power consumption of integrated circuits is becoming more of a concern of designers. Power optimization is performed at different levels of a system design while trading-off various design parameters like supply voltage, well bias voltage, transistor sizing, circuit style, and micro-architecture, for example. One power management technique is to lower a supply voltage because this may give more than a linear savings in power. However, an increase in leakage power with scaling can result, which has led to dynamic voltage and threshold scaling (DVTS) where both supply voltage and substrate biases (e.g., threshold voltages) of a circuit are controlled to reach a power optimum point (POP). Power savings in computationally intensive circuits, such as for example, motion estimators and moving picture experts group (MPEG) codecs, which have significant fluctuations in their activity and performance requirements, can be improved with DVTS, for example.
For power optimization during super-threshold operations of transistors, a ratio of active power to leakage power can be close to constant at a power optimum usage. Systems with DVTS have been implemented by maintaining the constant power ratio over a range of operating frequencies. However, the DVTS implementation may not measure active power or leakage power of the actual system in order to maintain the constant power ratio. Instead, DVTS implementations usually measure the active power or leakage power of the actual system indirectly by using mimic circuits. But, enabling the mimic circuit to track the power consumption of the actual system across process, voltage, temperature and activity variations can be difficult due to the complex nature of the actual system. For example, the actual system can be very complex and may include hundreds of thousands of logic gates (of all variations) configured in a complicated network. The mimic circuit would need to be smaller to reduce overhead and costs, and creating a smaller mimic circuit that will accurately model power dissipation of the actual system can be difficult and inherently error prone due to differences in scale and complexity of the two circuits. Furthermore, power dissipation of the actual system may vary due to input data patterns, and attempting to replicate such variation within the mimic circuit can lead to many errors.
Additionally, active power mimic circuits may only work in super-threshold operation because in a sub-threshold operation (and particularly weak inversion region operations), the active power to leakage power ratio at optimum power varies from constant. For example, when supply voltages are reduced to small values (e.g., essentially lower than the threshold voltage of the transistor), transistors in the circuit will operate in a sub-threshold regime in which the active power to leakage power ratio at optimum power varies from constant. However, when the supply voltage is higher than the threshold voltage, the operation will be in a super-threshold regime.
In deep-sub-micron technologies that have supply voltages as low as 1 volt, for example, digital circuits can span operation from super-threshold to near/sub-threshold regions for wide ranges of performance specifications.